refactor(jobs): address code review feedback on fp.job.step (Task 1.6)

- I1: Replace 'Task 1.6' markers in stub method comments and
  NotImplementedError messages with forward-looking phrasing.
  Task 1.6 is what just shipped (the field expansion); the action
  stubs are deferred to an unspecified future task. Stale markers
  would have confused future readers/operators.
- I2: Add test_cost_total_recomputes_when_rate_changes — insurance
  test that verifies @api.depends('cost_per_hour') triggers through
  the related-from-work_centre chain. Catches future Odoo upgrades
  that break related-depends.

Manifest 19.0.8.5.0 → 19.0.8.5.1.

Part of: native job model migration (spec 2026-04-25)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating/__manifest__.py

@@ -5,7 +5,7 @@
 
 {
     'name': 'Fusion Plating',
-    'version': '19.0.8.5.0',
+    'version': '19.0.8.5.1',
     'category': 'Manufacturing/Plating',
     'summary': 'Core plating / metal finishing ERP: facilities, processes, tanks, baths, jobs, operators.',
     'description': """